1. Field of the Invention
The present invention relates to electrical power generation systems and methods of operating electrical power generation systems in which a plurality of electrical power generators are arranged on a common electrical distribution arrangement.
2. Description of Related Art
Electrical distribution arrangements are known in which a number of electrical generators are provided and which supply a common electrical power network. This electrical power network may be associated with a number of loads such that variations in voltage and/or current must be accommodated in order to meet minimum or desirable electrical generator system levels. With regard to aero engines it is known to provide generators which are embedded within the engine in order to provide electrical power for local engine control loads as well as other electrical demands within an aircraft incorporating the aero engines. Each engine in the aircraft may incorporate its own generators and therefore a number of generators are provided or coupled to a common distribution arrangement. Parallel operation of the generators can bring benefits in terms of system efficiency, weight and availability of electrical power throughout the network. In order to achieve parallel operation of the generators a suitable control method is required such that conflicts in control of each generator do not occur. Normally it is required that one of the generators operates with voltage control and the other generators operate with current control.
Referring to the illustration marked FIG. 5, it will be noted that steady operation is illustrated by point 1 where there is a stable voltage Vcmd and a stable electrical current Icmd. In order to provide such stability as indicated above, one electrical generator is designated to be under the dynamics of voltage control. In such circumstances a comparator 2 compares the desired voltage of Vcmd with the output voltage Vout from the voltage controller and generator 3. If the voltage output Vout does not equal Vcmd then through appropriate configuration of the associated generator dynamic adjustment is made until there is parity. Other generators provide an electrical current Icmd which again is compared with the output current Iout in a comparator 4. Any disparity is adjusted through reconfiguration using an electrical current controller and an electrical power generator 5 in order to achieve parity between Icmd and Iout. In such circumstances the steady state point 1 depicted graphically should be maintained. It will be understood that different electrical current controlled generators may have different values of Icmd.
Voltage control and electrical current control are normally independent controls. Both controls may be implemented on the same generator but they will not be active at the same time. A generator operating under voltage control is responsible for controlling electrical system dynamics whereas a generator operating under electrical current control does not provide any dynamic support to the generator that is controlling the generator system voltage.
It will be appreciated that an electrical generator or generators under electrical current control will at least attempt to maintain its electrical current output at the demand level at all times. The electrical current demand is normally provided from a higher level of system control which monitors, or predicts the total load level in the system and then decides the current demand for each generator under current control. Based upon this principle, the electrical current demand is relatively slow changing and hence, the electrical current control has a relatively slow response time. In such circumstances it is not possible for full system dynamics to be fulfilled with traditional electrical current control regimes. Electrical generators under electrical current demand controls will not respond to any system dynamics. Nevertheless, with regard to some electrical distribution systems relatively heavy electrical loads will result in highly dynamic scenarios with regard to the electrical power generation system. For example within an aeroplane there may be regenerative electrical surface actuators. When a surface actuator is activated it will draw a dynamically changing electrical current. As the load is stopped, the energy stored in the mechanical system will be released back into the power system as regenerated energy. Due to this variability it is difficult for a controller to predict electrical load and electrical demand and therefore dynamic loads such as actuators will always pose generator system instabilities.
As indicated above generally voltage controlled generators will normally operate within stability margins up to a limit of load changes that can be managed before unacceptable voltage dynamics occur. When a heavy actuator load is switched into a power generation system or particularly a power distribution arrangement, the dynamics imposed can be too fast and too severe for the voltage controlled generator to maintain the system voltage within acceptable levels. In such circumstances unacceptable system operation is provided.
A further problem associated with established electrical current control techniques is that unpredictable over voltages may occur through disconnection from parallel operation with the electrical generator controlled by voltage. When system connection between the voltage control generator and the current control generator is lost, as a result of a fault or malfunction, the part of the load that is still connected with the current controlled electrical generator may be smaller than the generator output current rating. Such a situation will cause the voltage within this part of the system to rise when the same electrical current level is pushed through the lower rating load. In such circumstances an over voltage protection function is required whilst conversely an under voltage may occur with heavy load conditions which would cause an immediate shutdown of the electrical voltage generator.